A SELECTION FEOM THE OBSEEVATIONS MADE WITH THEM. 691 
librium. Lateral motion, however, must always exist in the different positions of the 
telescope, owing to the elasticity of materials ; and it must act injuriously in some 
degree, in proportion to the force required to move the speculum on the balls. Great 
care has been taken to make the fittings of the ring in which the speculum hangs as 
perfect as possible, and to connect its joints and bearings with the iron carriage, so as to 
reduce the lateral motion of the speculum to the smallest possible quantity. We have 
not tried Mr. Lassel’s ingenious ari’angement for relieving the edge pressure. Unless 
there were holes half through the speculum the experiment could not be fairly tried. 
Our 3-feet specula are also suspended in a ring, and are supported on fewer points by the 
lever apparatus, which, at a slight sacrifice of theoretical accuracy, has been thrown into 
one plane. We have rarely perceived any fiexure of importance, except where the action 
of the levers had been impeded by rust ; but the 3-feet specula, which weigh about thir- 
teen hundredweight, are very much stifier than the 6-feet specula, as is obvious on com- 
mon mechanical principles. Upon the whole, I am inclined to think there is a better 
prospect of improUng the definition of very large specula by increasing the original 
stifihess, than by endeavouring still further to eliminate slight disturbing forces. 
The 6-feet specula were ground with an iron tool, divided into squares, precisely as 
the 3-feet specula. The squares were larger, about 2 inches each side, and were not 
formed by cutting but by casting. A tool cast from the same pattern was employed as 
a polisher, but the surface of the squares was cut up by turning, so as to leave no more 
than half an inch of continuous surface. The tools weigh about one ton one hundred- 
weight each, and the hron is so disposed in them as to produce the utmost amount of stiff- 
ness. The photograph supplies all details (Plate XXIV. figs. 3 & 4). 
In grinding, about two-thirds of the weight is at first taken off by a counterpoise acting 
through a system of levers attached to it in thirty-six points, on the same principle as the 
levers which support the speculum. As the jDrocess proceeds the contact becomes more 
general, the friction increases, and there is more heat developed ; therefore the counter- 
poise is increased, till towards the conclusion the unbalanced weight of the grinder is 
reduced to about two hundredweight. Notwithstanding the great strength of the tool, 
we found that if after the grinding was over it was suspended by its centre, the flexure, 
after a week or two, became so great that on replacing it on the speculum and regrind- 
ing with it, the action commenced at the edge ; it is therefore always, when not in use, 
suspended by its levers. The curvature of this tool was adjusted in the ordinary way by 
gauges. These, as they were to be employed in the adjustment of the speculum to 
focus, were made with great care. One side of each gauge was first made into a straight 
edge, by the well-known scraping process of Mr. Whitworth, and the two were then 
very slightly ground for a few minutes with fine emery to remove the marks of the 
scraper, but no more. Ordinates were then set out, an inch apart, and marked to the 
calculated length by means of an instrument applied to the straight edge, very similar to 
the joiner’s gauge, but made of brass, with a fine scale and vernier. Through the extre- 
mities of the ordinates the curve was traced by means of a steel point, guided by a 
